1. Technical Field
This disclosure relates generally to an image forming apparatus, and more particularly to an image forming apparatus including multiple carriages each having a recording head in which positions of the carriages are reliably controlled.
2. Description of the Background
Related-art image forming apparatuses such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions include inkjet-type image forming apparatuses employing a liquid discharge recording method. Such inkjet-type image forming apparatuses include a recording head composed of a liquid-droplet discharge head that discharges droplets of a recording liquid such as ink. The droplets of the recording liquid attach to a recording medium such as a sheet of paper or the like to form an image on the sheet while the sheet is conveyed.
In a full-color inkjet-type image forming apparatus capable of forming full-color images, a carriage having a recording head composed of a liquid discharge head that discharges ink droplets of four or more colors, that is, for example, black (K), yellow (Y), cyan (C), and magenta (M), or a recording head having nozzle arrays, scans to form full-color images on a sheet. However, when monochrome images are mainly formed by such a full-color inkjet-type image forming apparatus, the recording head for discharging color ink droplets can dry out, causing nozzles of the recording head to clog. Although color ink droplets that are not used for image formation are discharged from the recording head to clear the recording head for the purpose of preventing clogging of the nozzles, such discharge is a waste of color ink. Consequently, even monochrome images may not be formed during monochrome image formation because the apparatus runs out of color ink.
Published Unexamined Japanese Patent Application No. H02-001327 (hereinafter referred to as JP-H02-001327-A) discloses a serial printer including a first carriage having a group of recording heads for discharging color ink droplets and a second carriage having a recording head for discharging black ink droplets. During color printing, the first and second carriages are connected to each other to integrally scan and perform printing. By contrast, during monochrome printing, only the second carriage scans and performs printing operations, and the first carriage is moisturized in order to prevent the group of recording heads from drying.
In the configuration in which multiple carriages are connected to or disconnected from each other as disclosed in JP-H02-001327-A, the accuracy with which each carriage is positioned can greatly affect printing accuracy. Specifically, improper connection of the carriages or relative displacement between the carriages connected to each other due to repeated connection and disconnection of the carriages can cause a shift in printing ions of the recording heads, possibly resulting in irregular printing including uneven image density and so forth. However, how to improve positional accuracy of each carriage is not disclosed in JP-H02-001327-A.
Published Unexamined Japanese Patent Application No. H09-109423 (hereinafter referred to as JP-H09-109423-A) discloses a technique for controlling positions of multiple carriages which are connected to each other to integrally scan. Specifically, a carriage for discharging black ink droplets and a carriage for discharging color ink droplets can be selectively coupled with a scanner capable of scanning along a guide shaft through a gripper. A lock part provided to the scanner engages grip parts respectively provided to the carriages to lock the scanner and the carriages together.
In another approach, Published Unexamined Japanese Patent Application No. H09-240097 (hereinafter referred to as JP-H09-240097-A) discloses a technique for obtaining an amount to correct looseness of connection between carriages and scanner. Specifically, a carriage for discharging black ink droplets and a carriage for discharging color ink droplets are selectively coupled with the scanner through a gripper, and sensor shielding plates are respectively provided to the scanner and the carriages. The amount to correct looseness of connection between the carriages and the scanner is obtained based on the timing of the sensor shielding plates shielding an optical path of a home position sensor provided at a predetermined position of the apparatus.
However, although multiple carriages are connected to the scanner through an intermediate member such as a gripper according to the techniques disclosed in JP-H09-109423-A and JP-H09-240097-A, the intermediate member may prevent the positions of the carriages from being accurately controlled. Further, because the shielding plates are respectively provided to the carriages to detect the positions of the carriages according to the technique disclosed in JP-H09-240097-A, displacement between the carriages may not be accurately detected.
In yet another approach, a technique for correcting a printing position by accurately detecting a position to where liquid droplets are discharged (hereinafter referred to as a landing position of liquid droplets) and correcting deviation in the landing position of the liquid droplets has been disclosed in Published Unexamined Japanese Patent Application No. 2008-229917. Specifically, an image forming apparatus includes: pattern forming means for forming, on a water-repellent member, an adjusting pattern constituted as multiple independent liquid droplets discharged from a recording head; a pattern reading sensor composed of a light emitting element for emitting light to the adjusting pattern and a light receiving element for receiving regular reflection light from the adjusting pattern; calculating means for calculating an amount of deviation in the landing position of the liquid droplets based on a result detected by the pattern reading sensor, and correction amount calculating means for calculating a correction amount of timing to discharge liquid droplets based on the amount of deviation in the landing position of the liquid droplets calculated by the calculating means.
However, although the deviation in the landing position of the liquid droplets can be accurately corrected by the above-described image forming apparatus, the image forming apparatus includes only a single carriage. Therefore, a displacement between multiple carriages caused by repeated connection and disconnection of multiple carriages is not solved by the above-described image forming apparatus.